This invention relates to a saw chain for cutting wood, e.g. trees and logs, which saw chain includes a guard feature that inhibits the occurrence of kickback, and more particularly wherein the guard feature is provided on the center links of the saw chain preceding the saw chain cutters or cutting links.
A popular saw chain used on consumer-type chain saws is a xe2x85x9cxe2x80x3 pitch chain having a low profile and provided with side links having a guard feature. The guard feature is provided above the body portion of certain of the side links and extends substantially the length of the body portion and somewhat rearwardly of the body portion. Thus, a following side link cutter is substantially protected from taking an excessive bite or penetration into the wood, particularly when there is contact between the chain at the upper quadrant of the bar nose and the material being cut (when cutting with the nose) or accidental contact with nearby branch or the like. Such contact may induce kickback.
Although popular, this chain is perceived to cut less efficiently due to the presence of the guard feature and, from the manufacturer""s point of view, is more costly to produce than is desired.
A cost issue results from the significant number of parts that have to be produced and assembled for the prior chain. The parts include: 1. a left-hand cutter; 2. a right-hand cutter; 3. a standard center drive link; 4. a standard tie strap (that fits on either side of the chain); 5. a right-hand guard side link; and 6. a left-hand guard side link. It is desirable to provide a center drive link with a guard portion rather than a side link in that this reduces the number of parts from six to five (the center guard link replacing both left and right-hand side guard links).
Cutting speed is believed to be affected by the length and height of the guard portion, in that the gaps between the cutting teeth of the cutting links carry the chips that have been cut and when this space fills up (partially due to the presence of the guard portion), the cutting teeth are forced away from the kerf bottom, i.e. out of the cutting mode. However, it is known that a reduced, e.g., shortened guard portion for the side link (front to back) is not as effective in preventing kickback. Particularly during a nose cut, the depth gauge of the cutting link as well as a shortened guard portion will be pressed into the kerf bottom, thereby compressing the wood which allows the following cutting tooth to penetrate further into the kerf and take the undesired excessive bite that can cause kickback. The longer guard portion requires a greater pressure to compress the wood and, thus, more effectively guards against the undesired excessive bite problem.
The present invention provides a guard portion at least at the trailing portion of the center link and because the cutting link and center link are overlapped (they share a common pivotal connection), the upwardly extended trailing guard portion is positioned alongside the depth gauge of the cutting link. This double thickness of depth gauge and guard portion is believed to more effectively resist penetration into the wood fibers of the kerf bottom (as compared to an elongated single thickness) and substantially enhances the resistance to excessive penetration of the following cutting tooth. In the preferred embodiment, the guard portion is relieved in the center area forward of the trailing guard portion to provide added chip carrying capacity, and then the center link is provided with a leading guard portion formed to provide a ramping effect that assists in resisting cutting tooth penetration but without unduly restricting chip-carrying capacity.
In the preferred embodiment, the configuration of the leading and trailing portions of the center link are cooperatively formed so that the ramp of the leading portion ramps the wood being cut in a direction that projects above the leading edge of the trailing portion, the leading edge of the trailing portion being itself shaped to avoid presenting a corner that might dig into the kerf.
The invention will be more fully appreciated upon reference to the following detailed description of the preferred embodiment having reference to the accompanying drawings. The cutting chain therein described has been found to be a lower-cost chain construction while providing improved cutting performance and without sacrificing safety.
A prior art patent that discloses a guard portion overlapping with the depth gauge is U.S. Pat. No. 4,425,830. However, the overlapping guard portion is provided on a preceding side link, the overlying guard portion extended rearwardly of the rear rivet of the side link which precedes the forward rivet of the cutting link. As will be noted from FIG. 3 thereof, the rearwardly extended guard portion pivots upwardly as the chain traverses the nose of the chain saw. Such upward pivoting inhibits the ability of the cutter to achieve a desired cutting penetration during nose cutting.